The active center of iron proteins is studied by 57Fe Mossbauer spectroscopy and magnetic resonance. Our aim is to establish correlations between spectroscopic and structural features and to elucidate the physical basis of the functions performed by the proteins. We concentrate on the ESR-silent high-spin ferrous iron for which only strong-field, low-temperature Mossbauer spectroscopy can provide detailed information about the electron ground state. Specifically, the following systems are studied: a. Cytochrome P450cam from the camphor hydroxylase of Pseudomonas putida, a monoxygenase that closely resembles mammalian cytochromes of the P450 type. b. L-tryptophan-2, 3-dioxygenase from Pseudomonas acidovorans, an allosteric enzyme with two exchangeable heme groups and two copper atoms. c. Compounds I and II of horseradish peroxidase and chloroperoxidase which are claimed to have the heme iron in the Fe(IV) state. d. Photosynthetic reaction centers in the native and reduced state. BIBLIOGRAPHIC REFERENCES: P. M. Champion, E. Munck, P. G. Debrunner, T. H. Moss, J. D. Lipscomb and I. C. Gunsalus. The Magnetic Susceptibility of Reduced Cytochrome P450cam. Biochim. Biophys. Acta 376, 579 (1975). P. G. Debrunner, C. E. Schulz, G. Feher and M. Y. Okamura. Mossbauer Study of Reaction Centers from R. Spheroides. Biophys. J. 15, 226a (1975).